Error indicating system



May 17, 1955 J. E. SHEPHERD ERROR INDICATING SYSTEM Original FiledApril: 30, 1942 4 Sheets-Sheet l May 17, 1955 J. E. SHEPHERD ERRORINDICATING SYSTEM Original F'iled April 30, 1942 4 Sheets-Sheet 2INVENTOR. J4/w55 E. Sf/:P//-RD /m Arron/ver Pf 012 on a N um E z 7. e 3Ncs os u C0 P r R E m 7 7 www ww /3 0 m M May 17, 1955 J. E. SHEPHERD2,708,751

ERROR INDICATING SYSTEM Original Filed April 30, 1942 4 Sheets-Sheet 3INV EN TOR. AL//wfs E. SIL/[mwen T TOR/VE Y May 17, 1955 J. E. SHEPHERDERROR INDICATING SYSTEM 4 Sheets-Sheet 4 my f6'.

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INVENTOR United States Patent O ERROR usInIcATrNG SYSTEM James E.Shepherd, Hempstead, N. Y., assignor to The Sperry Corporation, acorporation of Delaware Original application April 30, 1942, Serial No.441,188. Divided and this application March 31, 1948, Serial No. 18,159

16 Claims. (Cl. 343-16) The present invention is concerned withradio-directed lire control systems especially adapted for use inaircraft and against other fast moving aircraft.

The present application is a divisional of copending application SerialNo. 441,188, tiled in the United States Patent Office on April 30, 1942,now Patent No. 2,617,982.

For the protection of large aircraft, such as heavy bombers, it is knownto use flexible gun turrets movable independently of the craft inassociation with a computing gun sight or computer which is manuallytracked with the target and thereby derives the proper gun aiming datafor controlling the gun turrets. Up to the present time, however, suchinter-aircraft fire control devices, and also anti-aircraft re controldevices, have relied upon visual tracking of the target for determiningvthe correct gun aiming angles. Such prior art systems are subject tothe well known limitations of visual sighting, such as reliance uponproper weather and visibility couditions, upon sufficient lighting, andupon the restricted range of optical telescopes. Even under optimumconditions of visibility, the visual detection of the approach ofaircraft and visual tracking with aircraft have been difficult anduncertain. For instance, aircraft approaching from the direction of thesun can be seen only with the greatest difficulty. Furthermore, theobservercannot scan the whole zone of danger quickly and carefully withcertainty by the eye alone.

In order to overcome these and other disadvantages of the prior systems,the invention of parent application Serial No. 441,188 now patent2,617,982, provides a system in which the target is detected, located,and tracked by a radio beam which effectively replaces the Avisual lineof sight of prior systems. However, before describing the presentsystem, certain essential requirements for such a system will bediscussed.

Firstly, the defending aircraft must be appraised of the presence andapproximate direction or orientation of -all targets in its.vicinity inorder to be able to eifec tively plan and accomplish its defense. Inaddition, it is desirable that the approximate range of each of thesevarious targets should be indicated simultaneously with its location,for similar reasons. After having been warned of the presence,orientation, and range of these targets, and after'having chosen one ormore of them as of greater importance for immediate engagement, it isnecessary for the particular target selected to be tracked by the lirecontrol system in order to determine the target present position, suchas defined by its elevation, azimuth, and range, in the present case,and to determine the rate of change of position, as defined by targetelevation rate and azimuth rate, in order that the correct gun aimingangles for controlling the guns and turrets may be derived by thecomputer.

in order to relieve the tire control ofcer of asmucli of the burden oftracking as is reasonably possible, it is desirable to automaticallytrack with the target, at least in elevation and azimuth, and possiblyalso in range, so

2,708,751 Patented May 17, 1955 as to automatically set into thecomputer mechanism the proper target position and target rate data.

The system` of said parent application Serial No. 441,188 now Patent2,617,982 offers an improved type of warning or searching system for usein combination with several types of tracking or fire control systems.Preferably, since space and weight are at a premium in aircraft, thesevarious systems are combined as much as possible to use a minimum amountof equipment.

Accordingly, there is provided apparatus for indicating the presence,approximate orientation, and aproximate range of any selected or alltargets Within a predetermined portion of space, such as a hemisphere,which apparatus may be converted upon selection of a particular target,as shown in parent application Serial No. 441,188 now Patent 2,617,982,into any one of three different types of tracking systems: (l) a systemin which the fire control oliicer actuates the computer setting in sucha manner as to maintain a radio line of sight in track with a target,(2) a system in which a radio line of sight is automatically trackedwith a target and the tire control officer actuates a computer tomaintain it in synchronism with the radio line of sight, and (3) a fullyautomatic system in which a radio line of sight is automaticallymaintained in synchronism with the target and serves to automaticallyset into the computer the proper target data required by the computer.

By such a system both the warning and tracking may be performed entirelyindependently of any optical visibility conditions and at a much greaterrange than was formerly possible, without impairing in any way any ofthe desirable features of former types of tire control systems.

In addition, the operation of the system is made to agree insubstantially all operations to be performed with the operation of priorsystems and the natural instinctive reactions of the operator areutilized by the provision of controlling operations which are naturallydictated by the circumstances encountered.

Accordingly, it is an object of the present invention to provideimproved gun control systems independent of the visual devices.

lt is another object of the present invention to provide improveddevices for indicating the relative displacement between the orientationof a distant object and a predetermined axis such as a radio line ofsight.

More specifically, it is an object of the present invention to providean error indication on a cathode ray tube by intensity modulation.

Another object resides in the provision of means whereby anverror andthe direction of such error is provided by intensity modulation of thebeam of a cathode ray tube describing a predetermined geometric figure.

Another lobject resides in the provision of an error indicating systememploying a cathode ray tube in which a circle is described by the beamof the cathode ray tube and the beam is intensity modulated whereby toprovide a crescent-like indication of the direction of such error.

Still another object resides in providing' an error indicating system inwhich the magnitude and direction of an error are represented on thescreen of a cathode ray tube by shifting the geometric figure producedon the screen of said tube from the electrical center of said tube in adirection corresponding to the directionof the error and to an extentrepresentative of the magnitude of said error and simultaneouslymodulating the intensity of the beam in each cycle thereof forming theiigure additionally to represent said error by the relative positions ofthemore and less intense portions of the iigure. I

Other objects and advantages of the present invention will becomeapparent from'the following specification and drawings, in which:

amarsi Fig. l shows a block or flow diagram of the System of theinvention during searching operations.

Fig. 2 shows a corresponding block diagram of the system during manualtracking operations.

Fig. 3 shows a corresponding block diagram of the system during manualautomatic operations.

Fig. 4 shows a block diagram of the system during full automaticoperations.

Fig. 5 shows a schematic perspective view of one form of scanner usefulin the present system.

Fig. 6 shows the radiation pattern of the directive antenna array usedwith the scanner of Fig. 5.

Fig. 7 shows a longitudinal cross-sectional view of the radiationpattern of the scanner of Fig. 5 during any of the tracking operations.

Fig. 7A is a cross-section of Fig. 7 taken along lines 7A-7A thereof.

Fig. 8 shows a schematic block wiring diagram of one form of radiotransmitting, receiving and indicator circuit for searching operations.

Fig. 8A shows a representative view of the cathode ray screen of theindicator of Fig. 8.

Fig. 9 shows a schematic circuit diagram of the spiral sweep orreference voltage generating apparatus for the circuit of Fig. 8.

Figs. 10A, 10B, 10C, and 10D are voltage-time graphs useful inexplaining the operation of the circuit of Fig. 9.

Fig. 11 shows a modification of a portion of the circuit of Fig. 9 tothe right of line A-A thereof.

Fig. 12 illustrates the phase relation of the circular deection voltagesderivable from the apparatus of Figs. 9 and 11.

Fig. 13 shows a block circuit diagram of one form of indicator usefulduring tracking operations.

Fig. 13A shows a representative indication produced by the circuit ofFig. 13.

Fig. 14 shows a rnodilcation of the indicating system of Fig. 13.

Fig. 14A shows a representative indication produced by the circuit ofFig. 14.

Fig. 15 shows a modification or" the portion of the circuit of Fig. 14to the right of line C -C thereof.

Fig. 16 shows another modification of the circuits of Figs. 13, 14, and15.

Fig. 16A shows a representative indication produced by the circuit ofFig. 16.

Fig. 17 shows a block circuit diagram of a further indicating systemuseful during tracking and incorporating several of the features ofFigs. 13 to 16.

Fig. 17A shows a representative indication produced by the circuit ofFig. 17.

Although we have herein described our invention in connection with a guncontrol system and particularly in connection with a more comprehensivesystem, more fully described in parent application Serial No. r441,188now Patent 2,617,982, it is to be understood that our present inventionis not necessarily limited to such use but may be employed in otherindicating systems involving different parameters.

As discussed above, the system, with which the present invention isadapted for use, is adapted for two major types of operation, namely,(l) a searching operation for roughly indicating the position and/ ordistance of any targets within the iield of operations of the device and(2) a tracking operation in which a particular target may be selectedand followed by the device for properly directing a gun thereat. Threealternative types of tracking operation, known as manuahsenti-automatic, and full automatic tracking may be used.

For describing generally these various types of operation, recourse ishad to Figs. 1 4, more specific details of the system being describedwith respect to later figures.

Fig. 1 shows a block or flow diagram of the present system whenoperating during searching. in this systern, a scanner 1 projects asharply directive beam of radiant energy, such as 19 in Fig. 6, obtainedas from a suitable transmitter and directive antenna arrangement 5. Thisbeam comprises a periodic sequence ot' short duration pulses of highfrequency energy, and during searching is swept in a spiral cone over apredetermined solid angle, which is preferably substantially ahemisphere, in such manner that the radiant energy is projected at sometime during its cycle into every part of the solid angle. Should anyobject or target be located in this solid angle, the projected radiantenergy will be reflected therefrom when the beam is directed thereat,and will be received in the same antenna system 3, which acts dually asa transmitting and a receiving system.

This reflected series of pulses of high frequency energy is received ina radio receiver 4 Whose output actuates a suitable indicator 6. Thisindicator, as will be described below more in detail, is preferably acathode ray tube whose electron beam trace is caused to sweep in spiralsin synchronism with and instantaneous correspondence with the spiralscanning motion of the scanner. For this purpose the indicator 6 is alsocontrolled from scanner 1. The received reected pulse is caused tomomentarily brighten the trace of the beam and thereby produce on thecathode ray screen an indication of the existence and approximateorientation of the reecting object. The approximate range of thereflecting object may also be shown.

The orientation of the scanner 1, which may be taken to be theorientation of the polar axis of the spiral conical scanning motion, isplaced under the control of a computer 7, whose elevation and azimuthsettings may be manually actuated from a suitable manual control 8.Computer 7 is adapted to calculate the proper gun aiming angles forintercepting the target by a projectile when the computer is set inaccordance with the present target position data, such as elevation,azimuth and range of the target, and in accordance with the rate ofchange of the present target position, such as elevation rate andazimuth rate. A suitable form for such a computer is shown more indetail in copending application Serial No. 411,186 now abandoned, forInter-Aircraft Gun Sight and Computer, filed September 17, 1941, in thenames of C. G. Holschuh and D. Fram. As is shown in this copendingapplication, the range setting of computer 7 may be performed by asuitable foot pedal 1t). The orientation control is eifected by a handlebar control 8 whose displacement about two independent axes represents acombination of the displacement and rate of change of displacement o fazimuth and elevation settings of cornputer 7, providing aided tracking.In operation, the controlling oicer actuates control 8 so as to maintainthe present target position setting of the computer 7 in track with thetarget, as evidenced (in the prior application) by a suitable opticalsighting arrangement. By so doing, the proper Atarget elevation, targetazimuth, target elevation rate and target azimuth rate are set into thecomputing mechanism 7 ltogether with the range data set in by foot pedal10, whereby computer 7 may determine the gun aiming angles. in thepresent system, the same operations are performed, but utilizing adifferent type of indicator to show the proper tracking conditions, aswill be described.

The scanner 1 is suitably controlled, as will be seen hereinafter (inaccordance with the target elevationand target azimuth setting ofcomputer 7. The gun aiming angles determined by computer 7 are used tosuitably control the orientation of one or more guns or'turrets 9, whichare thereby rendered effective against the target.

A suitable type of gun control apparatus for orienting the guns 9 underthe control of the computer 7 is shown in copending application SerialNo. 424,612, for Hydraulic Remote Operating Systems, led December 27,1941, in the names of E. L. Dawson, F. M. Watkins and C. N. Schuh, Ir.,now Patent No. 2,445,765. 'Itis to be noted that the present system isnot conned to the use of this particular type of gun control apparatus,but that any other suitable type of Vremote-control; system may also beused. If desired, the guns 9 needV not be directly controlled fromcomputer 7 but may be locally controlled in accordance with'suitableindications transmitted from computer 7 in any well known manner.

The system as shown in Fig. 1 is not intended for use as the actual guncontrol system but is merely intended to search out possible targets andto enable the scanner to properly locate a target for the purpose oflater tracking with it. For this reason, the control from computer 7 toguns 9 is shown dotted in Fig. 1. After a target is observed on thescreen of cathode ray indicator 6, the manual control8-of computer 7 isactuated to adjust the orientation of scanner 1 to the position wherethis orientation -coincides as closely as possible with the orientationofthe desired target, as evidenced by the position of the bright spotindication on the indicator screen. When this adjustment has been made,the system is ready to change-over to' the tracking operation.

The system is adapted to use three separate and distinct types oftracking, any one of which may be selected at the option of the recontrol officer. t is to be noted that each of these types of trackingsystem may be used independently of the others if desirable. For all ofthese types of trackingoperation, scanner 1 is energized fromtransmitter 2 by the same type of periodic pulse wave as described withrespect to the searching operation. However, scanner 1 no longerperforms spiral scanning as in Fig. 1 but instead it is converted toperform a narrow circular conical scanning with a very small apex angle.Preferably, this angle is of the order of the angular width of theradiation and reception pattern derived from antenna 3, included inFigs. 6, 7 and 7A.

Thusif antenna system 3 is adapted to produce a beam of radiant energyhaving a directive radiation pattern such as V19 in Fig. 6 with adirectivity axis 21 then, during tracking, beam 19 will be rotated byscanner 1 about an axis such as 23 in Fig. 7, whereby directivity axis21 performs a conical motion about axis 23, which may be termed thetracking directivity axis since itis this axis which denesthe radio lineof sight, as will be seen. Preferably, radiation pattern 19 is made tohave a small apex angle such as of the order of 4 in angular widthbetween the half-power points. Then, during tracking, the cone describedby axis 21 would preferablyhave an apex angle also of the order of 4. Inthis manner, the useful portion of the radiant energy would be projectedover a conical solid angle having an 8 apex angle. Energy reected vfroman object or target within the iield of this radiant energy will bereceived by antenna arrangement 3 and led thereby to receiver 4 whoseouput actuates the tracking indicator 6 to indicate the relativedisplacement between the scanner orientation defined by axis 23 and theorientation of V.the target.

In the system of Fig. 2, manual actuation of computer control 8 servesto set-azimuth and elevation data into computer 7 and at the same timecontrols the orientation of scanner 1, as determined by axis 23, toassume the same azimuth and elevation as is set into computer 7, in thesame manner as described with respect to Fig. l. In effect theorientation of scanner 1 is made the same as the orientation of computer7, the latter term meaning the orientation corresponding to the azimuthand elevation data set into the computer mechanism.

Also actuated from receiver 4 is a range indicator 6". A matching indexis provided for indicator 6" as described in more detail in copendingapplication Serial No. 15,398 for Radio Range Indicating System, filedMarch 17, 1948, inthe names of J.- E. Shepherd and G. E. White, whichislplaced under the control of range pedal 10 serving also.

to'set range data into computer 7..

In operating the system of Fig. v2, the operator will, by hismanual.control t3, orient scannerA 1 until the tracking indicator 6' shows thatthe target orientation coincides with the scanner orientation. At thesame time, the operator actuates the range foot pedal 10 to match therange index to the indication produced by range indicator 6". When theseconditions obtain, and are maintained even during the motion of thetarget, the operator will know that the proper data is set into computer7 and that the guns 9 controlled from the computed output of computer 7are directed at the correct aiming angles to intercept the target with aprojectile, and he may therefore, by a suitable tiring key or control,tire at the target.

This system is known as manual tracking since the operator, through hismanual control 8 and foot pedal 10, directly actuates the scanner andcomputer 7 to track with the target as evidenced by indicators 6' and6". The scanner 1, in effect, operates to produce a radio line of sightin the same way as the sighting telescope in a conventionalanti-aircraft or interaircraft system operates to produce an opticalline of sight, to enable the computer 7 to track with the presentposition of the target, whereby the proper gun aiming angles aredetermined.

A second type of tracking operation is illustrated in Fig. 3 and istermed semi-automatic tracking. In this case the scanner 1, againperforming circular conical scanning as described with respect to Fig.2, is caused to automatically align its orientation with that of thetarget. This is done by using the reected pulses received from thetarget to actuate suitable servomotors for orienting the scanner, whichis thereby automatically oriented toward and tracks with the target. Thecomputer 7 is again manually controlled from controls 8, in thisinstance to follow and track with the orientation of scanner 1. Thus,tracking indicator 6 in this type of system serves to indicate thedisplacement between the orientations of scanner 1 and computer 7,andcomputer 7 is actuated' to maintain this computer error at zero.

' the proper gun aiming angles and effective lire may be obtained fromthe guns.

Fig. -4 shows the third or full automatic tracking system in which nomanual actuation is necessary. Here, scanner 1 is automatically orientedtoward the target,

` under the control of the output of receiver 4, as in Fig. 3,

and, in addition, the orientation of computer 7 is caused toautomatically follow the position of scanner 1 by a suitable servomechanism. In this manner, the proper target azimuth and elevation dataare set into the computer 7. The range adjustment of computer 7 is alsoautomatically performed by a range control 10 under the control ofreceiver 4. This system, however, does not obtain the target rates, thatis, elevation rate and azimuth rate, in the same manner as in Figs. 2and 3.

In the system of Fig. 4, it is necessary to determine elevation rate andazimuth rate by actually measuring the angular rate of motion of theazimuth and elevation input controls of scanner 1. This may be done inany well known way, such as is shown and described in U. S.

" Patent No. 2,206,875, for Fire Control Device issued typeof'scanner 1. Thus, the scanner 1 may comprise a July 9, 1940, in thename of E. W. Chafee et al. In this manner, all the required data may beset into computer 7 and therefore the guns 9 are automatically orientedat the proper gun aiming angles and automatically follow directiveantenna system 3, shown as comprising a parabolic wave guide reilector,and energized through suitable electromagnetic wave guide connections 11from transmitter 2. A suitable construction for scanner 1 is shown anddescribed in copending application Serial No. 438,388, for ScanningDevices, tiled April 10, 1942, in the names of L. A. Maybarduk, W, W.Mieher, S. i. Zand and G. E. White, now Patent No. 2,410,831. As thereindisclosed, the antenna arrangement 3 in one form may be continuouslynodded or oscillated at a slow rate about nod axis 12 which is itselfrapidly and continuously rotated or spun about spin axis 13 therebyproducing a spiral conical scanning pattern by the continuous wideningof the conical sweeping about spin axis 13. To convert from the spiralsearching scanning to the circular tracking scanning, the nod motionabout the nod axis 12 is interrupted with the orientation of thedirective radiation or recept-ivity pattern axis 21 displaced slightlyfrom the spin axis 13.

In order to feed radiant energy from wave guide il to the radiator 3,suitable stationary joints 14 and rotating joints 16 are provided asdescribed more in detail in the above-mentioned copending applicationSerial No. 438,388, now Patent 2,410,831, and in copending applicationSerial No. 447,524 for High Frequency Apparatus, tiled June 18, 1942, inthe names of W. W. Mieher and I. Mallet, now Patent No. 2,407,318.

To provide the necessary control of tracking indicator 6 from scanner 1,in the manner to be described, suitable self-synchronous positiontransmitters are provided for producing signals indicative of theinstantaneous position of the radiator 3 in nod and in spin, that is,indicative of the orientation of axis 21. The nod transmitter isindicated schematically at 17, the spin transmitter at 18. Thesetransmitters may be of the well known Selsyn, Autosyn, or Telegon types.

Referring to Fig. 6, there is shown the radiation or reccptivity pattern19, of the antenna array 3 of Fig. 5. It will be noted that thisradiation pattern 19 preferably is axially symmetrical about axis 21,and is substantially contained within a narrow solid cone 22, therebyforming a sharply directive beam of transmitted energy or a sharplydirective reception pattern. Pattern 19 has been exaggerated forpurposes of illustration, and preferably is very narrow, such as about 4between the half-power points. During searching operations the axis 21of this beam 19, by virtue of the combined effect of the nodding andspinning action of scanner 1, is caused to sweep out a spiral cone inspace, the solid angle of this sweep being suitably chosen and rangingup to a complete hemisphere as desired. Preferably, the angular pitch ofthis spiral is chosen to be of the order of the effective angular widthof the vbeam 19 whereby, during one complete spiral scan every portionof the conical solid angle will have had radiant energy projected to it,and radiant energy may be received from every such portion. The rates otnod and spin of the scanner of Fig. are suitably chosen to pro vide asufficiently short time interval for a complete scan, suitable for thepurposes at hand.

During tracking operations the nod motion of scanner 1 is stopped at aposition so that the axis 21 of maximum radiation or receptivity isdisplaced slightly from the spin axis 13 about which the radiationpattern 19 is rotated. ln this way, as shown in Figs. 7 and 7A, energyoi constant intensity is radiated or received along an axis 2.3coincident with spin axis 1,3. However, along some other axis, such as2d, for example, maximum radiation and maximum receptivi re encounteredonly once during each spin cycle, resulting in a spin frequencymodulation of waves received by reflection from an object oriented alongaxis 24.

The use of the same antenna arrangement for transmitting and receivingincreases the sharpness of the Vresulting determinations since theover-all response pat tern is the product of the radiation andreceptivity patterns. lf desired, however, a non-directional transmitter or receiver could be used with the described scanner actingrespectively as a receiver or transmitter.

Conversion from searching to tracking scanning is effected, as describedin application Serial No. 438,388, now Patent 2,410,831, merely byenergizaton of a suitable control solenoid. Other types of scanners arealso described therein, requiring different apparatus for convertingfrom searching to tracking, but all adapted to be used for searching ortracking in the same manner as the scanner' of Fig, 5.

it may also be desirable to adjust the axis o f this spiral scanningduring the searching operation. For this purpose, scanner 1 may beprovided with an elevation axis 26 and an azimuth axis 27 about which itmay be suitably adjusted, in the manner described in application SerialNo. 438,388, now Patent 2,410,831, the control action being as describedbelow. Also, suitable elevation and azimuth position transmitters 28 and29 may be used, as will aiso be described below.

Fig. 8 shows one form of radio and indicator system for giving suitableindications during searching. Thus assuming that the scanner or" Fig. 5is performing the spiral scanning described above, antenna array 3 isfed with radiant energy as over wave guide 11, from a transmitter andmodulator unit 31. This transmitter 31 is adapted to produce highfrequency radiant energy in any `vell known manner, and to modulate thishigh frequency energy oy means of periodically recurring short durationpulses such as may be derived from a conventional control oscillator andpulse generator 32. There is thus radiated from the radiatingarrangement 3 a sequence of short pulses of high frequency radiantenergy. The freq uency of control oscillator 32 and thereby therepetition frequency of the radiated pulses is chosen to have a suitablyhigh value such that a substantial number of pulses is sent. ont duringeach spin rotation of the seanner 1 oi Fig. 5. Suitable values forvarious constants of thc circuits during this form of operation havebeen found to be the foliowing: spin rotation, 1200 revolutions perminute; nod oscillation, 30 complete oscillations per minute; pulserepetition frequency, 2G00 per second. With these values it will be seenthat one complete cycle of spiral scanning will be accomplished each twoseconds, one second being taken up in a spiral scan from zero nod tofull nod, the other second of the cycle comprising the time for spiralscanning from full nod back to zero nod. During each half of thecompletecycle 20 complete spin rotations are performed; Thus,

for a ful hemisphere of scan, the angular advance for` each spin cyclewill he approximately 41/2 degrees, which is of the order of magnitudeof the width of the radiation pattern .19 shown in Fig. 6. The pulserepetition rate of 2000 pulses per second gives pulses per spinrotation, which thereby produces one pulse for each 3.6 degrees ofmotion of the radiation pattern 19 during scanning. Since the radiationpattern 1 9 is approximately 4 to 5 degrees wide, it will be seenthat'at least one pulse of radiant energy is transmitted to each pointof the hemisphere.

Should a distant object be in the field of the system during radiation,at least one pulse will be incident thereon, and reilected therefrom.This rellected pulse o r pulses will be picked up in the antennaarrangement 3 and conducted through Wave guide 11 to the receiver unit 4through aT- R box 33. T-R box 33 is adapted to pass the relatively lowintensity received pulses but to block out the relatively high intensitytransmitted pulses derived from transmitter 31. A suitable forni forsuch a T-R box 33 is shown in copending application Serial No. 406,494'for Radio Apparatus for the Detection and Location of Objects, tiledAugust l2, 1941, in the names o f J. Lyman et al., and comprises, as istherein shown, anionizable medium containing a spark gap within aresonant cavity which is resonant to the high frequency of transmission.The

' spark gap is so adjusted that the low intensity received waves areinsufficient to create a discharge across the gap, whereas the highintensity transmitted pulses are sufficient to create such a discharge,which thereby ionizes the ionizable medium and effectively shortcircuits the wave guide 11 to these transmitted waves. In this mannerthe receiver unit 4 is effectively isolated from the high intensitytransmitted pulses while being free to receive the pulses reflectd froma distant object. Receiver unit 4 includes conventional pre-amplifying,detecting and wideband amplifying units, all well known in the art, andis adapted to produce, in its output, signal currents or voltagescorresponding to the wave shape of the envelope of the receivedreflected wave.

The received pulses are applied to the control grid 37 of the cathoderay tube indicator 6 shown in Fig. 8. Grid 37 is provided with asuitable bias, as by way of lead 38, such that, with no output fromreceiver 4, the cathode ray beam, produced by the usual means, isprevented from reaching the screen of the cathode ray tube indicator 6.However, this bias is also so adjusted that the received pulses 36derived from the receiver unit 4 are permitted to momentarily render theelectron beam trace visible on the screen of indicator 6. Thus, it willbe clear that each time a reected pulse is received a momentary brightspot occurs on the cathode ray screen.

In order to give an indication of the orientation of the reflectedobject with respect to the location of the system ofthe invention it isdesirable to produce a spiral scanning of the electron beam insynchronisrn with and corresponding instantaneously to the spiralscanning of the radiation and reception pattern 19. Suitable devices forobtaining deflecting voltages which will produce such a spiral scanningare shown in Figs. 9 through l2. Assuming, for the moment, that suchspiral sweep voltages, designated as P1 and P2, have been obtained,these voltages P1 and P2, to be hereafter described more in detail, areimpressed upon respective pairs of deiecting plates of the cathode rayindicator 6 and produce a spiral scanning of the electron beam such thatat each instant the orientation of the latent trace of the beam on thescreen of the cathode ray indicator 6 with respect to the screen centeror pole 39 of Fig. 8A, cor-responds to the instantaneous orientation ofthe beam axis 21 of antenna array 3 of scanner 1. Under theselconditions the momentary brightening or intensifying of the electronbeam under the control of receiver 4 will produce `a momentary brightspot such as 41 shown in Fig. 8A. If a plurality of objects havingdifferent orientations are within the effective field of the searchingsystem further bright spots such as 42 and 43 will also be produced,each having an orientation with respect to pole'39 respectivelycorresponding to the orientation of the corresponding reecting objectwith respect to the spin axis 13 of the scanner 1.

As described above, the transmitted pulses and hence the reflectedpulses are of quite short duration, such as the order of '1 microsecond.In order that vthe bright spots 41, 42 and 43 may be more clearly shownit is desirable to let the beam impinge upon the screen for a longerinterval. For this purpose a signal storer 44 is inserted betweenreceiver 4 and intensity control grid 37. This signal storer 44 maysimply comprise a condenserresistor network adapted to beinstantaneously charged by a pulse derived from receiver 4 and lwhichwill maintain its charge beyond the duration of the pulse. However, thetime constant of the signal storer 44 'is preferably so chosen that thisaccumulated charge will be fully dissipated within a time not rnuchlonger than one recurrence period of the transmitted pulses in orderthat erroneous indications shall not be obtained. In this way the traces41, 42, 43 are made brighter. In addition, the screen of indicator 6 ispreferably made of high reten. tivity, so as to maintain its indicationfor a substantial interval after excitation is removed. l Y

Fig. 9 shows one form of circuit for producing the spiral sweep voltagesused with indicator 6 of Fig. 8. In

aymara '1 l0 this figure, nod transmitter 17 is indicated as being of-atwo-phase type having a single-phase energizing winding 46 and atwo-phase secondary winding 47, in this in? stance connected in seriesto provide a single output. Winding 46 is energized from a suitablesource.48 of alternating current. The output voltage appearing acrossthe polyphase winding 47, namely voltage V1 having wave shape as shownin Fig. 10A, will therefore be an alternating voltage having thefrequency of source 48 and an amplitude varying in correspondence withthe amount of nod, referred to the orientation of the scanner spin axisas Zero nod. This wave is shown in Fig. 10A, being illustrated as havinga linear change of amplitude with nod. It is to be noted that ordinarilythis change of amplitude will be sinusoidal in character. However, bythe use of proper motion converting `devices whereby full nod motioncorresponds to a small angular displacement of winding 46 with repect towinding 47, it may be made, linear as illustrated; Preferably full nodismade to correspond to less than 45 rotation of transmitter 17, resultingthereb)l in substantially linear output as shown in Fig. 10A.

During searching operations, switch 49 will be connected to terminal 8and hence the output voltage V1 of nod transmitter 17 is fed to thesingle-phase winding 51 of the spin transmitter 18. The output from eachof the two-phase windings 52 and 53 of spin transmitter 18 will then bethe wave of Fig. 10A sinusoidally modulated in amplitude at thefrequency of spin.` This is shown in. Fig. 10B for the winding 52. Thewinding 53, being displaced 90" in space with respect to winding 52,will have induced in it a voltage of similar wave shape but displaced 90in phase at the spin frequency. In effect, spin transmitter 18 serves asa two-phase generator of spin frequency whose output amplitude iscontrolled by nod transmitter 17.

To each of these voltage outputs from windings 52 and 53 there is addeda voltage of the frequency of source 48, as by way of transformer 54,producing the wave shown in Fig. 10C. It is to be noted that the wave ofFig. 10B represents in effect a suppressed-carrier modulated wave. Thereinsertion of the carrier as by transformer 54 produces the usualmodulated carrier wave l shown in Fig. 10C. The resulting two waves arethen rectified or detected in Vrespective rectitiers 56 and 57 andfiltered in filters 58 andv 59 to produce the output voltages appearingon output leads 61 and 62 having the wave shape shown in Fig. 10D,namely, phase-displaced volt` ages of spin frequency modulated` by thenod wave envelope. I

These two voltage appearing on lines 61 and 62 will be phase displacedby 90 of the spin frequency. They will be termed the spiral sweepvoltages P1 and P2, respectively. As is well known, if two voltages ofequal amplitude and frequency, phase displaced by 90, are irn-v pressedon the respective pairs of deecting plates of aA cathode ray tube, theresulting trace of the electron beam will be circular. By simultaneouslyvarying the amplii tudes of the two voltages the-diameter of the circlewilll be varied. f

In the present instance, by using the two waves P1 Vand Pz as thedeflecting voltages, the beam willbe caused to produce a circularpattern of constantly changing diameter and will thereby produce aspiral pattern similar to the pattern swept out in space by thescanner 1. It will, therefore, be clear that these voltages P1 and P2are particularly suited for use in indicator 6.

During any of the three types of tracking, nod transmitter 17 isdisconnected from spin transmitter 18 byA switch 49, which then connectswinding 51 of spin trans mitter 18 to a fixed source of alternatingvoltage, such'as source 48, as by way of lead 50.l .In thiscase,.outp.ut

sweep voltages P1 and'Pz will have'constant amplitude,

producing a circular trace kon indicator 6, and accordinglywill betermed circular sweep voltages.

avosn'ei type of spiral sweep voltages P1 and Pz will be obtained as inFig. 9.

Fig. 12 represents the sweep voltage P1 and P2 which are derived fromthe foregoing voltage generating apparatus when no modulationcorresponding to the nod motion of the antenna or transmitter iseffected. These voltages indicated as 70 and 71, respectively, havesubstantially equal maximum amplitudes and are of a frequencycorresponding to the spin frequency of the antenna, being phasedisplaced by 90. These voltages are therefore useful under trackingconditions, when conical scan is effected, as azimuth and elevationreference voltages in determining those error components of the angle oferrors between the directivity axis of the scanner and the directiontoward the chosen target. The spiral sweep voltages P1 and P2 will havethe appearance of that shown in Fig. D but phase displaced as in Fig.12.

During manual tracking, in which the scanner 1 and computer 7 aremanually actuated together, it is necessary to provide some type ofindication whereby the gunner may know when he is accurately trackingwith the target; that is, when scanner 1 is oriented toward the target,and the correct target orientation data is being set into computer 7.

Fig. 13 shows one type of such tracking indicator circuit. As describedabove, during all tracking operations, including manual, semi-automaticand full-automatic tracking, the transmitted beam is rotating about aVery narrow cone, as illustrated in Figs. 7 and 7A. If the target issituated exactly along the axis of symmetry 13 (or 23) of this cone, allreflected pulses will be received with constant and equal intensity.This indicates proper tracking with the target. Should the target,however, deviate from this desired condition, as when its orientation isalong axis 24 of Fig. 7, the reflected pulses will periodically vary inamplitude at a frequency equal to the spin frequency of the scanning. Ineffect, this produces a spin frequency modulation upon the receivedpulse-s.

Furthermore, it will be clear that the instantaneous maximum of thisspin frequency modulation will occur at the instant the axis 21 of thebeam is projected closest to line 24. Hence the phase of this modulationbears a relation to the spinning of the scanner which is indicative ofthe orientation of the reflecting object. Accordingly, the orientation of the object with respect to spin axis 13 (0r 23) may be determined by.comparing the phase of the modulation with the spin cycle of thescanner or, which is the same, with the sweep voltages P1 and P2, whichwill be of fixed amplitude upon operation of the switch to cut out unit17 and have fixed phase relation to the scanner motion.

Thus, referring to Fig. 13, the receiver output 94, now comprising thereceived pulses periodically varying in amplitude at the spin frequency,is applied to a further detector, such as 167, including a lter adaptedto pass waves only of the spin frequency, whose intensity wiil thereforegive a measure of tne amount of deviation of the target from the desiredorientation with respect to the tracking system, (within a predeterminedregion) and whosegphase with respect to -voltages P1 and P2 indi- Catesthe -relative orientation of the target with respect' to the scannerorientation. Preferably, detector 107 also acts to fill in the waveenvelope between pulses,

whereby a substantially sinusoidal output in phase with the pulsemodulation is obtained.

Such phase comparisons are performed in the respective azimuth andelevation phase sensitive amplifiers 108 and 109 in Fig. 13. Theseamplifiers are of any conventional type adapted to produce in theiroutputs a undirectional voltage corresponding in polarity and magnitudet'o the sense and magnitude of the component of the output of detector107 co-phasal or anti-phasal with the reference voltage P1 or P2.

The respective outputs of am-plifiers 108 and 109 are connected to therespective deecting plates 99, 101 of indicator 6 and will therebyproduce on its screen a bright dot, such as 111, shown in Fig. 13A,whose position relative to the pole 39 of the screen is the same as theposition of the reflecting object relative to the axis of the conicaiscanning performed by the scanner 1 and hence indicates the displacementor "error between scanner and target. It will be clear that the firecontrol ofiicer, to perform accurate tracking, must control the settingsof the computer in such a manner that spot 111 is maintained at the pole39, in which case the scanner is oriented toward the target and theproper azimuth and elevation data corresponding to present targetposition are introduced into computer 7. If, at the same time, the rangecontrol 1t) of the computer 7 is adjusted, the complete data required bycomputer 7 is thus set into it, and the output of computer 7 may seiveto correctly orient the guns or turrets 9 to effectively engage thetarget. Y

Fig. 14 shows a modification of the circuit of Fig. 13 adapted toproduce as its indication a small circle which is displaced from thepole 39 in the same manner as spot 111. ln this modification, thedeflecting plates 99, 101 of indicator 6 are actuated in the same mannerdescribed with respect to Fig. 13. In addition, however, indicator 6' isprovided with the usual deflecting coils, such as coils 112, therespective pairs of these coils being energized from the respectivecircular sweep voltages P1 and P2. The effect of these defiecting coils112 is to produce a circular motion of the cathode ray trace. The effectof the deflecting plates 99, 101 is to displace the center of thiscircle in accordance with the scanner error, yielding the indicationshown in Fig. 14A, comprising movable circle 1,13.

Fig. l5 shows a modification of that portion of Fig. 14 to the right ofline C-C. This modification is adapted produce the same type ofindication shown in Fig. 14A. In this instance, no extra deflectingcoils 1.12 are necessary, but the sweep voltages P1 and P2 are coupleddirectly to the deecting plates of the indicator V6' to be superimposedupon the output waves of the rcspective phase sensitive amplifiers 108and 109. Preferably, sweep voltages P1 and P2 are coupled to thedeflecting plates by Vway of coupling and blocking condensers 114, whichserve to isolate the two inputs to each pair of deflecting plates fromone another.

It will be clear that here also the gunner must operate the scanner soas to cause the circular indication 113 to be maintained symmetricalwith respect to pole 39 or to suitable cross hairs such as 116.

Fig. 16 shows a further modification of the tracking indicators of Figs.13 to 15. Here the electron beam of cathode ray indicator 6' is causedto trace a fixed circle, such as 117, on the screen of indicator 6. Theoutput 94 of receiver 4 is connected to a wide-band amplifier 110 of anywell known type adapted to reproduce the output wave of receiver 4 inamplified form. The output of amplifier is connected to the intensitycontrol grid 37 of indicator 6. When the scanner 1 is correctly orientedwith respect to the target; that is, -when the target orientation iscoincident with the scanner spin axis 13, each of the reflected pulseswill be of equal amplitude. Accordingly, each of the spot tracesproduced on the indicator screen at the instant that the reflected pulseis re- 13 ceived will be of equal intensity. Should the targetorientation differ from the scanner orientation, certain of thesereceived pulses will be of greater amplitude, resulting in thecorrespondingly greater intensity of their corresponding spot traces.Under such conditions, one portion of the circular trace 117 Will havegreater brightness than the diametrically opposite portion. In eifect,an arcur ate section of circle 117 Will have maximum brightness, whereasthe diametrically opposite arc will have minimum brightness. The angularposition of this bright arc with respect to cross hairs 116 willcorrespond directly to the angular position of the target with respectto the scanner. Accordingly, the operator then must actuate the scannerso as to produce equal intensity for all parts of the circle 116, inorder to eiect proper tracking.

The actual indication of Fig. 16A, instead of being a true circle, isreally a circular arrangement of dots. In the illustrative circuitvalues indicated above, there would be 100 of these dots in eachcomplete circle. If it is so desired, a signal storer, such as describedwith respect to Fig. 8, may be inserted to enhance the brightness ofthese spots. A further signal storer, having longer time constant, maybe used to run over between spots and provide a true circular indicationhaving increased brightness for parts thereof corresponding to thetarget orientation.

Fig. 17 shows a complete radio and indicator circuit combining severalof the features of Figs. 13 to 16. From what has been described above,it should be clear that the indication in this instance, as shown inFig. 17A, will be a circular trace 152 whose position with respect topole 39 represents the scanner error, this circular trace 1.52 beingmodulated in intensity inthe manner described with respect to Fig. 16.

In the foregoing, I have indicated that the cathode ray tube provides anindication of scanner error. As set forth in parent application SerialNo. 441,188 now Patent 2,617,982, the indication provided by the tubemay be of the error or relative displacement between the computer andthe scanner. Hence, it is to be understood that the indicating system ofthe present invention may be employed to indicate the direction of anymeasurable error, along one or two coordinates, and errors involvingvarious parameters and also the magnitudes of such errors and, in itsbroader sense, is not limited to the indication of any particular errorsuch as herein'shown and described mainly for exemplary purposes.-

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madeWithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown inthe accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimedis:

1. A tracking indicating system comprising means for transmittingelectromagnetic energy, means for receiving that portion of said energyreected from an object, one of said means having a highly directionalcharacteristic, means for causing rotation of said directionalcharacteristic about a predetermined axis, means for generatingsubstantially constant amplitude alternating voltages in phasequadrature and of the same frequency as the period of rotation of saiddirectional characteristic, a cathode ray tube, means energized by saidvoltages for causing the beam of said tube to rotate at a constantradius and in timed relation to the rotation of said directionalcharacteristic to thereby produce a circumferential trace forming acircle on the screen of said tube, and means responsive to the amplitudeof the modulation envelope of received energy for controlling theintensity of said circular trace to `indicate the relative displacementof said object and said axis.

`2. An error-indicating system comprising a rotatable energy-responsivemeans and means for producing a periodically varying signal voltagehaving a frequency depend- CSX ent upon the rate of rotation of saidrotatable means and having a time phase dependent upon and varying withthe angular position of an object about a reference point, means forrotating said energy-responsive means, a cathode ray tube, means forcausing the beam of the tube to describe a predetermined path in timedrelation to the rotation of said energy-responsive means, and means formodulating the intensity of said beam in accordance with said signalvoltage.

3. An error-indicating system comprising an energyreceiving means havinga directional axis and means for rotating the directional axis of saidmeans about a predetermined axis in a generally conical manner, areceiver connected with said energy-receiving means for producing aperiodically varying signal voltage having a time phase dependent uponand varying with the angular position of an object relative to saidpredetermined axis, a cathode ray tube, means for causing the beam ofsaid tube to describe a predetermined geometric figure on the screen ofsaid tube, and means for modulating the intensity of said beam inaccordance with said signal voltage.

4. An error-indicating system comprising scanner means rotatable about areference axis for providing a signal voltage cyclically variable withrotation of said scanner about said axis and having a time phasedependent upon and varying with the position of an object relative tosaid axis, means for rotating said scanner means, a cathode ray tube,means for causing the beam of the tube to produce a circular trace onthe screen of the tube in synchronism with the rotation of said scannermeans, and means for modulating the intensity of said beam in accordancewith said signal voltage.

5. An error-indicating system comprising means for producing aperiodically varying signal voltage having a time phase dependent uponand varying with the angular position of an object about a referencepoint, a cathode ray tube, means for causing the beam of the tube todescribe a predetermined path in cyclic synchronism with the signalvoltage, means for modulating the intensity of said beam in accordancewith said signal voltage, and means for deflecting the beam of said tubein accordance with said signal voltage.

6. An error-indicating system comprising scanner means rotatable about areference axis for providing a signal voltage cyclically variable withrotation of said scanner about said axis and having a time phasedependent upon and varying with the position of an object relative tosaid axis, means for rotating said scanner means, a cathode ray tube,means for causing the beam of the tube to produce a circular trace onthe screen of the tube in timed relation to the rotation of said scannermeans, means for modulating the intensity of said beam in accordancewith said signal vol-tage, and means for deilecting the center of thecircular trace from the electrical center of said tube in accordancewith said signal voltage.

7. A tracking indicating system comprising means for transmittingelectromagnetic energy, means for receiving that portion of said energyreected from an object, one of said means having a highly directionalcharacteristic, means for causing rotation of said directionalcharacteristic in a generally conical manner about and at a small angletoa predetermined axis, a cathode ray tube, means for causing the beamof said tube to describe a predetermined closed geometric figure intimed relation with the rotation of said directional characteristic, andmeans for modulating the intensity of the beam of said tube inaccordance with the amplitude of the modulation envelope of the receivedelectromagnetic energy.

8. A tracking indicating system comprising means for transmittingelectromagnetic energy, means for receiving that portion of said energyreflected from an object, one of said means having a highly directionalcharacteristic, means for causing rotation of said directionalcharacteristic about and at a small angle to a predetermined axis, acathode raytube, means for generating awww a beam-deecting voltage ofthe same frequency as the rotation of saidr directional characteristic,means controlled by said beam-dellecting voltage for causing the beam orsaid tube to describe a circular path in synchronism with the rotationof said directional characteristic, and means for modulating theintensity of the beam of said tube in accordance with the amplitude ofthe modulation envelope of the received electromagnetic energy.

9. A tracking indicating system comprising means for transmittingelectromagnetic energy, means for receiving that portion of said energyreilected from an object, one of said means having a highly directionalcharacteristic, means for causing rotation of said directionalcharacteristic in a cyclic manner about a predetermined axis, a cathoderay tube, means for generating a pair of beam-deecting voltages inquadrature phase relation and of the same frequency as the rotation ofsaid directional characteristic, means controlled by said quadraturevoltages for causing the beam of said tube to describe t a circular pathon'the screen of the tube, and means for modulating the intensity of thebeam of said tube in accordance with the amplitude of the modulationenvelope of the received electromagnetic energy.

l0. A tracking indicating system comprising means for transmittingelectromagnetic energy, means for receiving that portion of said energyreilected from an object, one of said means having a highly directionalcharacteristic, means for causing rotation of said directionalcharacteristic about a predetermined axis, a cathode ray tube, means forcausing the beam of said tube to describe a predetermined closedgeometric figure, means for modulating the intensity of the beam of saidtube in accordance with the amplitude of the modulation envelope of thereceived electromagnetic energy, eans for obtaining two voltagecomponents proportional respectively to the values of the amplitude ofthe modulation envelope of the received energy at fixed points spacedsubstantially 90 apart in the rotation of the directional charac.-teristic whereby to obtain measures along perpendicular coordinates ofthe direction of said object relative to the axis of rotation of saiddirectional characteristic, and means for deecting the beam of said tubeby said voltage components along corresponding coordinates of the tubescreen.

ll. A tracking indicating system comprising means for transmittingelectromagnetic energy, means for receiving that portion of said energyrellected from an object, one of said means having a highly directionalcharacteristic, means for causing rotation of said directionalcharacteristic about a predetermined axis, a cathode ray tube, means forgenerating a pair of beam-deecting voltages in quadrature phase relationand of the same frequency as the rotation of said directionalcharacteristie, means controlled by said quadrature voltages for causingthe beam of said tube to describe a circle on the screen of the tube,means for modulating the intensity of the beam of said tube inaccordance with the amplitude of the modulation envelope of the receivedelectromagnetic energy, means for obtaining voltage componentsproportional respectively to the values of the amplitude of themodulation envelope of the received energy at fixed points spacedsubstantially 90 apart in the rotation of the directional characteristicwhereby to obtain measures along perpendicular coordinates of thedirection of said object relative to the axis of rotation of saiddirectional characteristic, and means for deflecting the beam of saidtube by said voltage components along corresponding coordinates of thetube screen.

l2. A tracking indicating system comprising means for transmittingelectromagnetic energy, means for receiving that portion of said energyreflected from an object, one of said means having a highly directionalcharacteristic, means -for causing rotation of said directionalcharacteristic about a predetermined axis, a cathode ray tube, means forcausing the beam of said tube to describe a circular trace on the screenthereof in synchronism with the rotation of said directionalcharacteristic, and means for modulating the intensity of the beam inaccordance with the amplitude of the modulation envelope of receivedenergy.

i3. A tracking indicating system comprising means for transmittingelectromagnetic energy, means for receiving that portion of said energyreilected from an object, one of said means having a highly directionalcharacteristic, means for causing rotation of said directionalcharacteristic about a predetermined axis, a cathode ray tube, means forcausing the beam of said tube to describe a circular trace on the screenthereof in synchronsm with the rotation of said directionalcharacteristic, means for modulating the intensity of the beam inaccordance with the amplitude of the modulation envelope of receivedenergy, and means for deiiecting the center of said circular tracerelative to the center of said tube in a direction corresponding to thedirection of said object relative to the axis'of rotation of saiddirectional characteristic. Y

14. Apparatus for indicating the magnitude and sense ot the relativedisplacement of two axes comprising means for producing signal voltagesproportional to two components of said displacement measured alongrelatively angularly disposed coordinates, a cathode ray tube, means forproducing a trace of the cathode ray beam in the form of a circle on theface of said tube, and means responsive to said signal voltages fordisplacing -the center of said circular'tra'ce along two coordinates ofthe tube face corresponding to those of the signal voltage measures,

l5. Apparatus for indicating the direction of relative displacement oftwo axes comprising a cathode ray tube, means for producing a trace ofthe 'electron beam in the form of a circle on the face of said tube,means for detecting the direction of relative displacement of said twoaxes, means responsive to said detecting means for modulating theintensity of s aid circular trace to produce varying degrees ofbrightness along the circumference thereof whereby the arcuate portionof said trace having an intensity differing from the balance of saidcircular trace will represent with respect to the center of said circlethe direction of the relative displacement of the two axes.

16. Apparatus for indicating the magnitude and sense of the relativedisplacement'of two axes comprising means for producing signal voltagesproportional to two components o f said displacement measured alongrelatively-angularly displaced coordinates, a cathode ray tube, meansfor producing a circular trace of the electron cream of said tube, meansresponsive to said signal voltages for displacing the center of saidcircular trace along two coordinates of the tube face corresponding tothose of the signal voltage measures, and means -for modulating theintensity of said circular trace to indicate said relative displacement.

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